The present disclosure relates to nitride semiconductor light-emitting devices, specifically relates to nitride semiconductor light-emitting devices in which a protection film is provided on a light-emitting facet.
Among semiconductor light-emitting devices, semiconductor laser diodes are widely used as a light source for playback and record of optical disc media, such as a compact disc (CD), a digital versatile disc (DVD), and a Blu-ray Disc (registered trademark). Because high-speed information recording is required, higher output power of the semiconductor laser diodes is particularly necessary so that much information can be recorded in short time.
However, it has been generally known that if optical output of the semiconductor laser diode is increased, a phenomenon in which no laser oscillation occurs in the semiconductor laser diode occurs due to a degradation phenomenon in which an operating current gradually increases, or a phenomenon called optical damage. Thus, to increase the output power, techniques for reducing or preventing such degradation of the semiconductor laser diode are being actively researched and developed. In particular, sudden death due to optical damage occurs at a front facet, which is a light-emitting facet of a laser cavity. Thus, efforts are being made to increase strength and stability of the protection film which covers a facet of the cavity. In general, a facet of a cavity of a semiconductor laser diode is covered with a protection film to control a reflection coefficient at the facet of the cavity, and avoid adhesion of foreign substances and oxidation of the facet of the cavity.
FIG. 16 schematically shows a cross-sectional structure of a conventional nitride semiconductor laser diode disclosed in Patent Document 1 (Japanese Patent Publication No. 2007-318088). As shown in FIG. 16, the conventional nitride semiconductor laser diode includes a laser structure 100 in which an active layer (light-emitting layer) 102 is sandwiched between an n-type semiconductor layer 101 and a p-type semiconductor layer 103. A p-side electrode 104 is formed on the p-type semiconductor layer 103. An n-side electrode 105 is formed on the n-type semiconductor layer 101.
A protection film 106 made of a metal oxide such as silicon oxide (SiO2) or aluminium oxide (Al2O3) is provided on a rear facet which serves as a mirror of the cavity in the laser structure 100. Further, aluminum nitride (AlN) is used as a first protection film 107 provided on a front facet of the cavity, and aluminium oxide (Al2O3) is used as a second protection film 108 provided on the outer side of the first protection film 107.
It is considered that one of the reasons why semiconductor laser diodes with higher output power deteriorate is due to an interface reaction at the facet of the cavity, such as a solid phase reaction caused between the first protection film 107 etc. and the facet of the semiconductor by the heat generated by the laser structure 100 and the light absorbed by the laser structure 100, or oxidation of the facet of the cavity caused by the oxygen remaining in the protection films, or oxygen in the package, diffusing in the protection films concurrently with laser oscillation.
Further, heat generated by the laser oscillation may cause the protection films to separate from the laser structure 100, or cause cracks in the laser structure 100, and the laser structure 100 is physically broken. The first protection film 107 made of AlN, which is shown in FIG. 16 as a conventional protection film, is a crystalline thin film. Therefore, the first protection film 107 made of AlN is strong, having a high thermal conductivity, and a superior material as a facet coating film. Moreover, since both of AlN and a nitride semiconductor are a nitride, a solid phase reaction between the AlN and the laser structure 100 is reduced.
However, the film stress of the AlN film is high. Thus, cracks etc. may be easily caused by laser oscillation. Further, it has been known that a light absorption band exists in the AlN near the wavelength of 400 nm due to a defect or the like, and the light absorption at the time of laser oscillation is considered as generating heat and causing diffusion of oxygen.
In view of this, Patent Document 2 (Japanese Patent Publication No. 2008-182208) discloses an AlN film having a structure which reduces film stress, and a method for forming such the AlN film. In Patent Document 2, AlN films each having different crystal orientation are layered to reduce the film stress, and prevent cracks and film separation due to heat.
Patent Document 3 (Japanese Patent Publication No. 2008-147363) discloses a structure in which AlN doped with a rare earth element is used as a protection film to reduce diffusion of oxygen. After the AlN is doped with a rare earth, the rare earth element is coupled with oxygen. It is therefore possible to reduce diffusion of the oxygen. By using such the AlN film doped with a rare earth element, it is possible to prevent the oxidation of the facet of the cavity and the film separation of the facet of the cavity, and reduce optical damage and deterioration of the facet. As a result, it is possible to perform laser operation for a longer time, compared to the case in which a general AlN film is used.